1. Field of the Invention
The present invention relates to a method of die casting spheroidal graphite cast iron.
2. Description of the Related Art
Spheroidal graphite cast iron is also called “ductile cast iron” and “nodular cast iron” and contains graphite in a spheroidal form, so is remarkably higher in strength and ductility compared with another cast iron with no spheroidal graphite and features a higher strength and toughness comparable with cast steel.
In the past, spheroidal graphite cast iron had been cast by sand molds, but due to the gradual cooling of the molten metal, the crystallized spheroidal graphite became coarse and there were limits to improvement of the mechanical properties. Further, castings made by sand molds are limited in the accuracy of their shape and dimensions.
It has therefore been demanded to obtain spheroidal graphite cast iron products improved in mechanical properties or accuracy of shape and dimensions exceeding the limits due to such sand mold casting. To meet with this demand, experiments have been conducted on die casting spheroidal graphite cast iron. If using die casting, a far faster cooling rate can be obtained compared with sand mold casting, so the spheroidal graphite finely crystallizes and the cast structure as a whole also becomes finer, so it is possible to improve the strength and ductility and also improve the accuracy of shape and dimensions.
With die casting, however, formation of chill crystals (rapidly cooled structure made of cementite) was unavoidable due to the fast cooling rate. If chill crystals are formed, the hardness of the casting becomes higher, but the toughness ends up being deteriorated and in the final analysis excellent mechanical properties cannot be obtained by die casting. Therefore, for example, as shown by the method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-288716, post-treatment such as heat treating the casting to break down the cementite forming the chill crystals into ferrite and carbon etc. has been necessary.
Another important point has been that in the conventional method, there has been the major problem that formation of internal defects such as shrinkage cavities was unavoidable both when using sand molds or dies and therefore the fatigue strength declined. In general, castings are prevented from the formation of shrinkage cavities by more slowly solidifying the feeder than the product section and supplementing molten metal from the feeder to the product section.
Here, since cast iron expands in volume due to graphite crystallization at the time of solidification, the method has been proposed of constraining this expansion of volume to cause the generation of internal pressure in the cavity and using this internal pressure to prevent the formation of shrinkage cavities. Specifically, the strength of the sand mold has been increased or the sand mold backed up by a die (back metal shell) to constrain expansion of volume.
However, in these methods, since a feeder is used, the expansion of volume by the crystallization of graphite ends up being eased by the flow of molten metal to the not yet solidified feeder, so in fact not that much of an effect of generation of internal pressure due to the constraint of expansion is obtained. Further, with the back metal shell method, formation of the sand mold is difficult and the sand mold layer has to be made thicker, so cannot be effectively backed up by a die. The sand mold part ends up moving so again a sufficient effect of generation of internal pressure due to the constraint of expansion cannot be obtained.
On the other hand, as a non-feeder design, the product section and gate have been optimized in shape, but no measure has been taken to prevent the formation of casting defects by constraining the expansion of volume.